The present invention relates to track accessing arm movement in disc drive systems. More specifically, the present invention relates to a magnetic pole piece assembly that creates and distributes a permanent magnetic field which interacts with a transient magnetic field produced by an actuator coil. The interaction of the two magnetic fields causes the actuator coil to move in a path proximate to the magnetic pole piece. The actuator coil is connected to a first end of an E-block assembly. The E-block assembly includes a plurality of track accessing arms at a second end. A transducer head assembly is typically connected to a resilient member, such as a gimbal spring, which in turn, is connected to the second end of the E-block assembly. The transducer head assembly is used to write and retrieve data from concentric tracks on magnetic media discs.
The disc drive system selectively applies current to the actuator coil which causes a transient magnetic field to emanate from the coil. Applying varied amounts of current to the actuator coil allows the disc drive system to position and hold the transducer head assembly over selected concentric tracks on the magnetic media disc. The transient magnetic field produced by the actuator coil interacts with the permanent magnetic field contained within the magnetic pole piece. The interaction between the transient and permanent magnetic fields creates a force which moves the actuator coil and thereby moves the E-block assembly with its plurality of track accessing arms over the tracks on the magnetic media disc.
In a rotary actuator assembly, the interaction between the permanent and transient magnetic fields creates a torque which rotates the E-block assembly around an axis of rotation. The rotation of the E-block assembly moves track accessing arms and positions the transducer head assemblies over selected concentric tracks on the magnetic media discs.
In a typical short seek operation, a disc drive system accelerates the actuator with full current applied to the actuator coil until it decelerates according to a velocity profile as dictated by the servo control system. In longer seeks, the actuator accelerates with full current applied to the actuator coil until it reaches the maximum speed allowed by the servo system. It maintains this speed until it decelerates according to the velocity profile similar to a shorter seek. The velocity or deceleration profiles are stored in the memory of a microprocessor in the disc drive system and define the velocity of the transducer head as it moves across tracks during a seek mode. The deceleration profile controls the current needed to achieve the design velocity. Hence, during a track seek operation, the microprocessor will drive the actuator coil with a current dictated by the velocity demand profile, typically accelerating the transducer head to a maximum seek velocity, and then decelerating the head to hopefully bring it to a halt over the desired destination track.
In setting the varying levels of current as dictated by the velocity profiles, the strength of the permanent magnetic field contained within the magnetic pole piece must be determined. The magnetic flux density produced by the permanent magnets along an actuator coil path is a factor in determining the amount of current that should be applied to the actuator coil. At locations along the actuator coil path at which the permanent magnetic field is weak, greater current must be applied to create a stronger transient magnetic field to ensure that there is enough magnetic force to move the actuator according to the velocity profile.
A disadvantage of prior art magnetic block assemblies is that it is common for the strength of the permanent magnetic field to taper off near the ends of the permanent magnet. This causes the weakest concentration of flux density to be near the ends of the permanent magnets, which also corresponds to the ends of the actuator path. In order to have enough current overhead when following the deceleration profile at all locations along the actuator path, the deceleration profile must be set according to the weakest magnetic flux density along the actuator coil path which is typically at the ends of the actuator path.